Here are some Christmas albums and songs I've found that I've liked. In many cases, I think most of the songs on an album are ok, but I particularly like a few of them.
Albums (in no specific order):
1) Home for Christmas - BarlowGirl
Although I didn't notice that much harmony (something the band is well-known for), but the songs are still done well. BarlowGirl is my favorite Christian band (quite possibly overally favorite group), but I don't think I'm being too biased here.
2) Winter Magic - Hayley Westenra
3) Christmas...From the Realms of Glory - Bebo Norman
4) Peace on Earth - Casting Crowns
5) Noel - Josh Groban
Perhaps you'll agree with me that Josh Groban has one of the best voices of any male singer out there today, but even if not, you may find you'll like some of his music anyway.
6) X Christmas - Various Artists
7) My Christmas - Andrea Bocelli
Even if you don't care for Andrea Bocelli's style of music (classical/pop, etc), you might be able to appreciate his impressive voice.
Songs (also not in any specific order):
1) Veni Veni Emmanuel - Winter Magic - Hayley Westenra
2) Carol of the Bells - Home for Christmas - BarlowGirl
3) I Heard the Bells on Christmas Day - Peace on Earth - Casting Crowns
4) Carol of the Bells - X Christmas - August Burns Red
5) Mary, Did You Know? - X Christmas - Kutless
Also, Mannheim Steamroller and Trans-Siberian Orchestra have some cool "modern-sounding" Christmas songs.
Once again I'll sugest that you check out the free mp3s on Amazon. Right now there is a lot of free Christmas music from well-known artists such as BarlowGirl, Casting Crowns, Jars of Clay, House of Heroes, and many more, as well as non-seasonal music. Unfortunately, I haven't found a way to see, say, all "free Christian music" or "free Celtic music", or...